There are various systems in the related art which support the driver of a motor vehicle in keeping the vehicle in its lane. For this purpose, such systems apply directed steering forces to the steering system of the vehicle if the vehicle leaves a prescribed path of motion. These systems are also called LKS systems (LKS: Lane Keeping Support). The degree of automation in this connection ranges from systems, which by the application of artificial steering forces indicate to the driver how he would have to operate the steering in order to keep the vehicle in its lane, to LKS systems that guide the vehicle in its lane in a fully automated manner.
LKS systems, as in DE 101 14 470 A1, for example, include essentially a lane detection system such as e.g. a video system, using which the course of the lane ahead of the vehicle and the relative position of the vehicle (the amount of deviation and orientation) in the lane may be determined. If the path of motion of the vehicle deviates too much from a prescribed path of motion, then the system varies the support torque exerted on the steering and thereby produces artificial steering forces. These are so strong that the driver is able to detect them haptically, and they are directed in such a way that the driver changes the steering wheel angle in the direction of the setpoint path of motion. If the steering wheel angle is too small, then the driver senses, for example, an additional torque in the direction of the inside of the curve. If the steering wheel angle is too large, on the other hand, then he senses an additional torque in the direction of the outside of the curve.
FIG. 1 shows a steering system having an LKS functionality, which is capable of keeping a vehicle 8 in its lane 10 in a fully automated manner. The system as a whole includes a sensor system 7 situated on vehicle 8 for lane detection such as e.g. a video system, by which it is possible to ascertain the amount of deviation and the orientation of vehicle 8 with respect to its lane 10 (amount of lateral deviation, path deviation angle) as well as the course of the lane (curvature, change in curvature) in front of the vehicle. Lane detection system 7 here includes a video camera and a special image processing software, which ascertains the desired geometric data from the image data.
The geometric data as well as additional driving state variables such as e.g. the driving speed are supplied to a mathematical reference model 13, which calculates from this a reference steering angle δRef. This reference steering angle δRef is the steering angle that would have to be set in the steering system in order to keep vehicle 8 optimally in its lane 10. This value δRef flows into a downstream control loop 11, which has the task of controlling the actual steering angle δ to match the specified setpoint value δRef and thus to keep vehicle 8 in its lane (normally in the middle of the lane).
Control loop 11 includes a node 2 (adding node), at which a control deviation Δδ=δRef−δ of the control variable is calculated, a transmission element 3, which forms the controller of control loop 11, and a steering control element 4 having a torque control loop, which forms the control element of steering angle control loop 11. Elements 2, 3, 4a and reference model 13 are normally implemented as software modules in a control unit 12.
Controller 3 has a P-behavior and is designed in such a way that a system deviation Δδ may be controlled in a stable manner. Controller 3 here includes a characteristic curve, which produces a guide torque (signal ME) as a function of the system deviation, which is supplied to steering control element 4. Steering control element 4, which includes a controller 4a and an actuator 4b, converts the torque ME, depending on the control characteristics, into an actuating torque MA, which is then applied to the steering system. Actuating torque MA is superposed on torque MF applied by driver 1 on the steering wheel. This is represented by an additional adding node 5. The transmission characteristic of the steering system is finally represented by a block 6.
FIG. 2 shows a driving situation in which vehicle 8 drives straight ahead in its lane 9 and in the process deviates from the center of the lane 16. The LKS system in this case calculates a reference steering angle δRef, which is to return vehicle 8 to the center of the lane 16. In this state, the control system is at a point P of controller characteristic curve 14 shown on the right and produces a corresponding guide torque ME in the direction of the center of the lane 16.
Known LKS systems are usually designed in such a way that vehicle 8 is guided on a specified setpoint path of motion, normally the center of the lane. A disadvantage of these systems, however, is the fact that many drivers tend to drive through curves, not in the center, but near the inside of the curve, thus cutting the corner. Because known LKS systems are fundamentally designed to guide a vehicle in the center of the lane, a deviation results, when cutting corners, between the steering angle desired by the driver and the reference steering angle, and thus a correcting steering intervention results on the part of the LKS system toward the center of the lane. This is shown schematically in FIG. 3a. The guide torque applied by the LKS system in the direction of the center of the lane 16 is indicated by ME.
When deliberately cutting a corner, a driver perceives such a steering intervention by the LKS system as unpleasant and interfering.